Re: mlock() on DAX returns -ENOMEM

[Yigal Korman <yigal@plexistor.com>]

On Mon, Apr 13, 2015 at 3:56 PM, Kirill A. Shutemov
<kirill@shutemov.name> wrote:
> On Sun, Apr 12, 2015 at 03:56:33PM +0300, Yigal Korman wrote:
>> Hi,
>> I've tried to mlock() a range of an ext4-dax file and got "-ENOMEM" in return.
>
> Is it comes from mlock_fixup() or -EFAULT from GUP translated to -ENOMEM
> by __mlock_posix_error_return()?

It comes from GUP. An associate followed the flow from GUP to
vm_normal_page which returns NULL for VM_MIXEDMAP.

>
>> Looking at the code, it seems that this is related to the fact that
>> DAX uses VM_MIXEDMAP and mlock assumes/requires regular page cache.
>> To me it seems that DAX should simply return success in mlock() as all
>> data is always in memory and no swapping is possible.
>> Is this a bug or intentional? Is there a fix planned?
>
> I think it's a bug.
>
> But first we need to define what mlock() means for DAX mappings.
>
> For writable MAP_PRIVATE: we should be able to trigger COW for the range
> and mlock resulting pages. It means we should fix kernel to handle
> GUP(FOLL_TOUCH | FOLL_POPULATE | FOLL_WRITE | FOLL_FORCE) successfully on
> such VMAs.

writable MAP_PRIVATE seems to work OK already.

>
> For MAP_SHARED and non-writable MAP_PRIVATE we should be able to populate
> the mapping with PTEs. Not sure if we need to set VM_LOCKED for such VMAs.
> We probably should, as we want to re-instantiate PTEs on mremap() and such.
> It means we need to get working at least GUP(FOLL_POPULATE | FOLL_FORCE).
>
> In general we need to adjust GUP to avoid going to struct page unless
> FOLL_* speficly imply struct page, such as FOLL_GET or FOLL_TOUCH.
>
> Not sure if we need to differentiate DAX mappings from other VM_MIXEDMAP.
>
> Any comments?

Indeed, the issue is VM_MIXEDMAP and not DAX specifically.
What will happen for other users of VM_MIXEDMAP that, unlike DAX, do
swap pages and need some special callback to handle mlock?

>
>> Also, the same code path that is used in mlock is also used for
>> MAP_POPULATE (pre-fault pages in mmap) so this flag doesn't work as
>> well (doesn't fail but simply doesn't pre-fault anything).
>>
>> Thanks,
>> Yigal
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>
> --
>  Kirill A. Shutemov

Thanks,
Yigal

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